1. Field of the Invention
The present invention relates to the field of modems, i.e., modulator, demodulator sets for digital communication over phone lines, and more particularly, to full duplex modems wherein simultaneous bidirectional communication may be carried out.
2. Prior Art
Various types of modems for digital communication over phone lines are well known in the prior art. For such transmission, base band signals must be modulated so as to carry the information to be communicated in a frequency range within the frequency range of a conventional telephone system. For this purpose, various types of modulation and demodulation techniques are well known to provide for one way communication at any one time (i.e., half duplex) or for simultaneous bidirectional communication, i.e, full duplex operation.
One prior art modem or data set of particular interest to the present invention is the Bell System Data Set 212A, as the preferred embodiment of the present invention is intended for communication compatability with the 212A data set, when communicating with a prior art 212A data set as either the originating or receiving device. (Obviously the present invention is also intended for communicating with another unit in accordance with the present invention, utilizing the standard 212A format). Characteristics of the 212A data set are described in various publications such as, by way of example, the Bell System technical reference entitled "Data Set 212A Interface Specification Jan. 1978", American Telephone and Telegraph Company publication No. 41214.
The 212 data set is basically a bilingual data set, having the capability of transmitting and receiving two kinds of line signals. One of the line signals is a frequency shift keyed format which allows the 212 to communicate with existing 300 baud data sets such as all of the switched network Bell 100 series data sets (this is commented on further in the Detailed Description of the Invention section). Of particular interest herein however, is the 212 data set capability of providing full duplex transmission and reception of serial binary data in a high speed mode at 1200 bits per second. In this mode, multisymbol signaling is used, specifically quaternary phase shift keying (QPSK) is used to transmit 1200 bits per second at a baud rate of 600. QPSK can also be represented as a form of quadrature amplitude modulation (QAM). For this purpose two bits at a time from a serial bit stream are encoded at the rate of 600 symbols per second, and quadrature amplitude modulated using a 1200 Hz carrier for transmission over the phone line from the originating data unit. At the same time the answering unit may be similarly gathering pairs of bits, coding the bit pairs at 600 symbols per second, quadrature amplitude modulating the coded signals using a 2400 Hz carrier and transmitting that information over the phone line from the answering unit to the originating unit. In this sense, the "originating" unit is the unit which initiates the establishment of the communication link over the phone line, and in accordance with the 212 format, sends at 1200 Hz and receives at 2400 Hz, whereas the other or answering unit receives at 1200 Hz and transmits at 2400 Hz. Each unit, of course, contains both a transmitter and a receiver, each capable of operating on either a 1200 or 2400 Hz carrier.
For detection of the received signal, the quadrature amplitude modulated signal must be demodulated by a synchronous demodulator, and since the transmitted signal is in effect a double sideband suppressed carrier signal, the carrier (1200 or 2400 Hz, as the case may be) must be reconstructed at the detector from the signal being received over the phone line. Further, the demodulated signal must be sampled at the rate of 600 samples per second at a rate phased to properly track and decode the data in the received signal. Obviously, methods are known in the prior art for recreating the carrier and establishing timing, though performance, cost and size can be substantially affected by the method and apparatus employed for this purpose.
In prior art 212 data sets both the transmitter and receiver portion of each unit generally may be characterized as having a large amount of channel filtering to remove both data signals from the other data band and as much noise outside the frequency range of interest as possible so that the output of the channel filters will be substantially composed of the modulated signal of interest. Two bandpass filters are normally used for these purposes, each typically 20th order pole filters of substantial cost and complexity. In the receiver the filtered signal is typically applied to an automatic gain control circuit to establish a predetermined signal level for demodulation by the synchronous demodulators. The outputs of the sine and cosine demodulators are then again relatively heavily filtered, partially for wave shaping and partially for removal of the extraneous frequencies introduced by the demodulation process itself. Thereafter the signal is sampled at the 600 sample per second rate and the transmitted bits are reconstructed, the carrier being reconstructed and timing established by various methods depending upon the particular data set. With this general form of construction, 212A data sets heretofore have been relatively large and expensive, and for these reasons have not yet been widely used in the presently burgeoning personal computer market, even though such use may be expected to greatly expand the usefulness of personal computers by giving them communication capabilities with large mainframes and data bases.